Fiber Bragg grating (FBG) has been accepted as an important sensor technology because of its self-referencing capability, large-scale multiplexing capability and immunity to electromagnetic interference. Wavelength-division multiplexing (WDM) technique [1] can be easily employed to multiplex and interrogate FBG sensor array, and thus it is commonly used in FBG sensor applications. The number of FBG sensors that can be accommodated in a WDM-FBG sensor array is determined by the usable spectral bandwidth of the system and the wavelength-shift of each FBG sensor. On the other hand, time-division-multiplexing (TDM) technique identifies the sensing signal by gating the pulses reflected from FBGs, therefore, FBGs having identical resonant wavelengths can be deployed along the same fiber. Hence, TDM-FBG technique relieves the spectral bandwidth issue and permits the interrogation of up to 100 FBGs along a fiber. However, the reflectivity of the FBGs employed in TDM sensor systems are generally less than 5% and thus the reflected signal power is fairly weak in comparison with WDM-FBG systems.
Various TDM systems have been reported during the last decade [2-7]. The main challenge of a TDM system is to measure the sensing signal accurately because of the weak signal power reflected from low reflectivity sensors. Amplified spontaneous emission (ASE) generated by an erbium-doped fiber amplifier [3] and passively mode-locked fiber laser operating in square-pulse regime [4] have been employed as sources to illuminate low reflectivity FBGs array with the objective to increase the signal power reflected from the FBGs. Another approach utilized active mode-locking technique [5] to selectively address individual FBG in a two-FBG array that act as reflectors of a linear cavity erbium-doped fiber laser. This active laser approach produces intense output power at the resonant wavelength of the selected FBG sensor. Recently, Lloyd et al. [6-7] reported a resonant TDM configuration that uses a SOA, a broadband reflector and an array of 10 FBGs to construct a linear resonant cavity sensor system, high power and high extinction ratio output signal were demonstrated by properly gating the SOA. While those prior practices are workable to various degrees, the need still exists for a simpler FBG sensor system with better performance and less construction expense.